A riboprinting technique based on the polymerase chain reaction and restriction fragment length polymorphism has been developed allowing positive identification of Entamoeba species. Using this technique we have confirmed the tentative identification of an Entamoeba found in the uterus of 1% of surveyed IUD users as E. gingivalis, a parasite of the human oral cavity. The question whether nonpathogenic (NP) and pathogenic (P) strains of E. histolytica (based on isoenzyme and repetitive DNA characteristics) constitute one or two species has been addressed. In vitro conversion of NP strains to P characteristics suggest there is only one species with two forms. We have found that riboprints of NP strains differ from those of P strains. However, riboprints of converted strains are indistinguishable from one form before, and the other form after conversion. This implies that E. histolytica is one species existing in two genetically distinct but interconvertible forms. Entamoeba histolytica-like isolates from humans, although different physiologically and biochemical from E. histolytica have been used in research as examples of avirulent forms of the latter. Riboprinting clearly indicates that E. histolytica-like amebae belong to a sub-group of E. moshkovskii, a species ordinarily isolated from sewage and considered to be free-living. Monoclonal and polyclonal antibodies developed in this laboratory were used to monitor differences between xenically and xenically cultured E. histolytica. Using monoclonal antibodies 2D7.10, 2F3.4, and a polyclonal antibody, 1125, which recognize a carbohydrate antigen, a polypeptide antigen, and hydrophobic membrane components respectively, we have obtained evidence suggesting bacteria can modulate surface antigens of E. histolytica. A cDNA expression library to study gene expression and control, and to obtain recombinant antigens for immunological studies has been prepared. Restriction fragments generated from a hypervariable region of E. histolytica ribosomal DNA have been shown to vary among clones of the same amebal strain.